What is a major sin?

One of the most comprehensive definitions of a major sin (kabeerah, pl. kabaa’ir) is that which describes it as: having any one of the following:

• it incurs a prescribed punishment (hadd) in this world,
• punishment in the hereafter is promised for it,
• Allah’s curse (or wrath) is on its perpetrator, or
• eemaan is negated from its perpetrator.

This definition is mentioned by many scholars including as-Saffaareenee [1] and others, and is put into verse by al-Hajaawee in Manzoomat-ul-Kabaa’ir:

And Allah knows best.

Reference:

[1] Ghidhaa’-ul-Albaab, 1/287